On 16th December, TYP03 released a new security update (TYPO3-SA-2010-022) for their content management system. Apparently, this web-based framework is widely used in many important websites.Within this update, TYPO3 team fixed a vulnerability that I've discovered a few weeks ago. In detail, this discovery pertains to a previous vulnerability fixed in TYPO3-SA-2010-020 and discovered by Gregor Kopf.

TYP03 decided to follow a policy of least disclosure. Although it's an Open Source project, no technical details are available in the wild besides these (1,2). As I strongly believe that this practice does not improve the overall security (as mentioned in a previous post), I've decided to briefly explain this interesting flaw.

From the advisory, we can actually deduce two important concepts:

A Remote File Disclosure vulnerability in the jumpUrl mechanism [..] Because of a non-typesafe comparison between the submitted and the calculated hash, it is possible [..]

In a nutshell, the JumpUrl mechanism allows to track access on web pages and provided files (e.g. /index.php?id=2&type=0&jumpurl=/etc/passwd&juSecure=1&locationData=2%3a&juHash=2b1928bfab)

The patch (see this shell script) simply replaces the two equal signs with three (loose vs strict comparisons).

That's the affected code:

Having this knowledge, it is probably clear to the reader that the overall goal is to bypass the comparison between $juHash and $calcJuHash. While the former is user supplied (string or array), the latter is derived from a substr(md5_value,10) (string).

In PHP, comparisons involving numerical strings result in unexpected behaviors (at least for me before studying this chapter).

If you compare a number with a string or the comparison involves numerical strings, then each string is converted to a number and the comparison performed numerically

If the string does not contain any of the characters '.', 'e', or 'E' and the numeric value fits into integer type limits (as defined by PHP_INT_MAX), the string will be evaluated as an integer. In all other cases it will be evaluated as a float.

Consequently, we can pad and wait till the substring of an md5 hash resembles this form. If you do the math, you will discover that the combined probability of having such calculated hash is considerably less than pure bruteforcing.

In practice, the number of iterations is even less as "0000E13131" and similar strings are also accepted.

To further improve this attack, I've discovered another bypass (TYPO3-SA-2010-022) which allows the disclosure of TYPO3_CONF_VARS['SYS']['encryptionKey']. In this way, it is possible to retrieve the key once and download multiple files without repeating the entire process. Using multiple requests, this attack takes a few minutes (8-20 minutes in a local network). A real coder can surely enhance it.

As you can see from the exploit (posted on The Exploit Database), the fileDenyPattern mechanism bypass is pretty trivial. A demonstration video is also available here (slow connection, sorry).

Keep your TYPO3 installation updated! A patch is already available from the vendor's site.

If you're a pentester, it's probably not news to you that "least disclosure" policies for disclosing vulnerabilities are fruitless. Unfortunately, they are even counterproductive for the entire security ecosystem and I will try to convince you within this post.

Before going any further, let's explain what "least disclosure" actually means.In a nutshell, least disclosure is about providing the least necessary facts of vulnerabilities that are needed to know if a user might be affected and what the possible impact would be. No technical details, no exploits, no proof-of-concept code.

As mentioned here, you may argue that it increases the overall security as a random "black hat needs to put some efforts in thinking and coding before he's able to exploit a vulnerability".

However, we all claim that "security through obscurity" is bad:

Aggressors don't have time constraints. They can analyze patches, read all documentation and spend nights on a single flaw

No technical details in the wild generally means no signatures and detectors in security tools